The method mostly accepted today for optical coherence tomography (OCT) is based on sweeping the frequency of a narrow band laser, usually termed as a swept source. Several principles of sweeping the laser emission have been developed. The most common principle employed is that of a spectral filter in a closed loop, where the tunable laser apparatus uses a Fabry-Perot filter, as disclosed in Patent Document 1 (U.S. Pat. No. 6,538,748 B1), a polygon filter, as disclosed in Patent Document 2 (U.S. Pat. No. 7,489,713 B2), or a micro electrical mechanical scanning (MEMS) filter, as disclosed in Patent Document 3 (U.S. Pat. No. 8,275,008 B2). Commercial vendors exist, such as Axsun and Santec, companies using MEMS filters. Such filters limit the tuning to several hundreds of kHz. In Non-Patent Document 1 (R. H. Huber et al, “Fourier domain mode-locking (FDML): A new laser operating regime and applications for optical coherence tomography”, Optics Express 14(8), 3225-3237 (2006)), a large tuning frequency has been reported using Fabry-Perot filters and principles of Fourier domain mode-locking (FDML) that allowed sweeping rates exceeding several MHz. However, the reliability of Fabry-Perot and the complexity of buffering limit the applicability of such principles. The filters mentioned above are based on mechanical movement of parts which limits their reliability. Therefore, there is an interest in akinetic laser sources that achieve tuning with no mechanical movement of parts. On the other hand, in modern applications of OCT there is an increasing demand in three dimensional imaging at high speed, with increased axial range.
An akinetic solution, based on the principle of dispersion tuning, is illustrated in the Non-Patent Document 2 (S. Yamashita, M. Asano, “Wide and fast wavelength-tunable mode-locked fiber laser based on dispersion tuning”, Optics Express 14(20), 9399-9306 (2006)). This method has limited tunability, due to the significant decrease in output power when driving it at high frequency repetition rates.
In Patent Document 5 (U.S. Pat. No. 8,605,768 B2), a solution to the previous problem is presented, which includes a positive dispersion region, a negative dispersion region and two modulators in a ring resonator, assembly which brings the amount of wavelength dispersion to approximately zero. This solution relies though only on externally driven modulation units, like acousto-optic or electro-optic modulators, for example.
In Non-Patent Document 3 (Y. Takubo, S. Yamashita, “High-speed dispersion-tuned wavelength-swept fiber laser using a reflective SOA and a chirped FBG”, Optics Express Vol. 21, No. 4, 5130-5139 (2013)), a wavelength swept fiber laser apparatus using a chirped fiber Bragg grating as dispersive medium is presented. Although the tuning wavelength range obtained is broad, the OCT imaging speed achieved was only up to 250 kHz, which is below the values demanded by modern swept source OCT applications.
In Non-Patent Document 4 (R. Stancu et. al, “Versatile Swept Source With Adjustable Coherence Length”, IEEE Photonics Technology Letters, Vol. 26, Issue 16, 1629-1632 (2014)), it is shown how the laser linewidth can be varied by driving the SOA in a dispersive laser cavity at different mode-locking frequencies. The sweeping frequency could not exceed a few tens of kHz.
Therefore, there is an interest in developing akinetic swept lasers that can address the disadvantages of the solutions presented above, first in providing much larger tuning speeds.
To make distinction between our invention and prior art, it is essential to note that the akinetic solutions presented above present the following characteristics. There are multiple values of the RF carriers that can be used, and the RF signal is tuned around such carrier values, differing by the inverse of the roundtrip. Around each such RF carrier, there is a tuning bandwidth and solutions presented above consist in tuning the RF signal within a single such band.
In the present invention, the RF tuning is practised over many such bands. More specifically, two resonant modulation effects are applied. A first modulation that induces mode-locking is imposed by driving the optical gain medium at a high radio frequency value. A second modulation is applied, inspired from the practice of Fourier domain mode-locking applied to Fabry-Perot lasers, where sweeping is performed at a rate close to the inverse roundtrip of the wave in the cavity. In opposition to the prior art where the sweeping has to be performed at the exact inverse of the roundtrip, the method disclosed here essentially uses a detuning of the excitation from the inverse of the roundtrip. Let us refer from now on to the inverse of the roundtrip as to the resonance frequency in the cavity, fR.
Essential for the operation of the akinetic laser according to the invention are two characteristics: (i) the frequency of the signal, fm, applied to mode-lock the laser is a large multiple N of fR and is deviated over a large tuning band, covering as many as b multiples of fR and (ii) the rate at which fS is tuned (the second modulation) is slightly detuned from the multiple M of the resonant frequency fR, where M is a small number, 1, 2, . . . 10. Due to the large deviation of fS, the characteristic (i), makes the invention different from the technology presented in the Non-Patent Document 2 (S. Yamashita, M. Asano), in the Non-Patent Document 3 (Y. Takubo, S. Yamashita) and in the Non-patent documents 4 (R. Stancu, David A. Jackson, Adrian Podoleanu) mentioned above. The characteristic (ii), due to the slight detuning from MfR, makes the invention different from the technology protected by Patent Document 1 (U.S. Pat. No. 6,538,748 B1), Patent Document 2 (U.S. Pat. No. 7,489,713 B2), and Patent Document 3 (U.S. Pat. No. 8,275,008 B2). By doing so, the driving method described in this invention can improve the scanning speed of an akinetic laser apparatus based on a dispersive cavity from hundreds of kHz to over several MHz.